This invention relates to a device for determining the charge condition of a battery, which device comprises:
a current-frequency converter for generating pulses at a frequency which is dependent on a current flowing through the battery;
computing means for determining a computed charge condition of the battery on the basis of the generated pulses; and
indicator means for indicating the computed charge condition as a fraction of the capacity of the battery. Such a device can be used in appliances fed by rechargeable batteries and is particularly suitable for use in a rechargeable electric shaver.
A device as described in the opening paragraph is known from DE-A No. 23 13 566. During charging and discharging of the battery the charge and discharge currents in this device are measured by means of a resistor which is arranged in series with the battery. The voltage across this resistor is amplified and subsequently integrated, with the amplification as well as the integration constant upon discharging being different from those upon charging of the battery. The integrated voltage is compared with a reference voltage and whenever this reference voltage is reached, a pulse is generated and the integrated voltage is reset again. These pulses are counted in a counter whose count is made visible by means of an indicator indicating the charge condition of the battery. While the battery is being discharged, a warning signal is given when a given count is reached and the load is decoupled after a certain time to prevent complete discharge of the battery. While the battery is being charged, the charge circuit is switched off when a maximum count is reached so as to avoid overcharging of the battery.
In the known device a modification of the capacity of the battery in the indicated charge condition is processed by measuring the external condition which causes the modification, for example, the temperature, and by adding extra pulses to or subtracting extra pulses from the count for this purpose.
This method leads to reasonable results if the capacity of the battery is substantially constant and if it only depends on one or at most several external conditions which can be subjected to parametrization. Consequently, the device is less suitable for use in a rechargeable electric apparatus in which nickel-cadmium (NC) batteries are used such as, for example an electric shaver. The capacity of such batteries is in fact not only dependent on the temperature but also on parameters such as age and history of use. For example, the capacity of an NC battery quickly decreases if the battery is not regularly charged and discharged completely. A condition such as the history of use can hardly be subjected to parametrization.